Reflective metal thin films are used in creating optical storage media. These thin metal layers are sputtered onto patterned transparent disks to reflect a laser light source. The reflected laser light is read as light and dark spots of certain length, converted into electrical signals, and transformed into images and sounds associated with music, movies, and data. All optical media formats, including compact disk (CD), laser disk (LD), and digital video disk (DVD), employ at least a single reflective metal layer, L1, for which aluminum is the metal of choice. More advanced optical media technology utilizes multiple reflective layers to increase the storage capacity of the media. For instance, many DVD's such as DVD 9, DVD 14, and DVD 18 contain two reflective layers, which enables two layers of information to be read from one side of the disk. The second layer, known as the L0 semi-reflective layer, must be thin enough, typically less than 10 nm, to allow the underlying L1 layer to be read, but it must still be sufficiently reflective, about 18% to about 30% reflectivity, to be read. The disk can further include one or more additional semi-reflective layers read from the same side as the L1 and L0 layers. The construction and reading methodology of a DVD containing two reflective layers is shown in FIG. 1.
When digital data is read from an optical storage medium, the lengths of the pits, typically of 9 different lengths, are read using an internal clock timing and converted into a high frequency electrical signal, which is truncated to generate square waves and transformed into a binary electrical data stream.
Variances in the length of the pits caused by molding the polycarbonate or the incomplete metallization of the entire pit can cause errors in interpreting the data reflected by the laser. For optical media applications, the electronic circuits that interpret the data are specially designed to allow for a certain number of errors. There are four primary error indicators for optical media data. These critical parameters are categorized as:
1) PI—the total number of unreadable pits within a specified area; while industry standards allow for 280 defects, many companies hold this parameter to a maximum of 100
2) Jitter—the timing variation in pit or land length compared to the internal clock pulses; the industry maximum is 8%
3) Reflectivity—the percentage of laser light reflected from the pits; the industry standard is 18 to 30%
4) I-14—the variance in the longest pit length; the industry standard is less than 0.15% within one revolution and less than 0.33% within the disk.
The initial quality of the master used for making the polycarbonate disks, the polycarbonate, and the reflective materials are critical to the production of accurate data. Not only must the metallizing material be capable of uniform deposition and reflectivity, it must also be capable of fully filling the data storage pits that store the data. In addition, the industry uses an environmental test that subjects the disk to a specific temperature and humidity for a specified period of time. The industry standard for this test is temperature of 70° C. at 50% relative humidity for 96 hours (70/50/96). Many companies have adapted stricter internal specifications to raise the temperature to 80° C. and humidity to 85% for 96 hours (80/85/96).
After manufacturing, the data storage disks are scanned for errors, exposed to the environmental testing chamber, and subsequently re-analyzed for errors. Any failures at any testing stage, based on industry standards for error rates, or marked deterioration, even if not actually failing, after environmental testing will lead to rejections. The environmental testing demands a corrosion resistant material for the reflective metallizations. While a thickness of 20 nm of Al generally is adequate for the fully reflective layer as produced, a thickness of 40 nm may be required to provide adequate reflectivity after environmental exposure. Typically, about half of the original aluminum layer is transformed into transparent aluminum oxide during this environmental test. The semi-reflective layer is dramatically more critical since its apparent thickness and reflecting qualities cannot change by more than about 10% of its original relative value during environmental exposure.
In addition to the testing noted above, there is also a non-industry specification regarding UV or sunlight exposure. It has been found that disks made with silver alloys can discolor when subjected to sunlight. While the chemistry of the reaction is not fully understood, it is caused by a combination of the silver alloy used for the semi-reflective layer and the adhesive used to attach it to the fully reflective layer. A disk is deemed to have failed once its reflectance falls below 18% for either the semi-reflective layer or the fully reflective layer, the latter being viewed through the adhesive and the semi-reflective layer.
Aluminum, gold, silicon and silver alloys have been successfully used to create reflective layers for optical storage media. Because of its low cost, excellent reflectivity and sputtering characteristics on polymeric materials, aluminum is an especially preferred metal for a reflective coating that is used almost exclusively whenever there is only one reflective data layer and is also used to form the fully reflective L1 layer on a two-layer DVD. However, aluminum oxidizes readily, and its reflectivity can be compromised upon environmental exposure. This oxidation prohibits the use of aluminum for all but the fully reflective layer, where it is deposited more heavily than the semi-reflective layer would allow. Gold and silicon were the first materials to be used for the semi-reflective layer in DVD construction, but both materials have significant drawbacks. Gold provides excellent reflectivity of red laser light, excellent sputtering characteristics, and superior corrosion resistance but is very costly. Silicon is also reflective and free from corrosion but does not sputter as efficiently as the other metals. Furthermore, silicon is brittle, and cracks may form during thermal cycling and mechanical flexing, which prevents delicate data from being read. U.S. Pat. No. 5,640,382 describes the construction of a DVD data storage disk, and U.S. Pat. No. 5,171,392 describes the use of gold and silicon for the semi-reflective data storage layer; the disclosures of these patents are incorporated herein by reference.
Silver, like gold, has excellent sputtering characteristics and reflectivity, but the corrosion resistance of pure silver is inadequate for it to be used as the semi-reflective layer. Considerable effort has been expended to make silver sufficiently corrosion resistant so that it can be used for the semi-reflective layer, as described, for example, in U.S. Pat. Nos. 6,280,811, 6,292,457, and 6,351,446, the disclosures of which are incorporated herein by reference. These patents describe silver based alloys for optical media whose corrosion resistance is improved by the addition of other precious metals such as palladium, platinum, and gold. While markedly less expensive than gold alone, the addition of these precious metals to silver in contents up to 30 wt. % dramatically increases their cost over that of pure silver.
For the manufacture of optical data recording and storage media, there is an ongoing need for silver alloys with uniform sputtering characteristics and improved corrosion resistance that do not require the inclusion of more expensive precious metals. This need is met by the alloys of the present invention, whose properties make them especially suitable for use in optical data recording and storage media, in particular, for use in the semi-reflective layer of a DVD.